In a field of image processing, there is a case where an graphic image drawn in a frame memory is temporarily saved to a display memory and is outputted to a display unit or the like from the display memory by transforming the graphic image into a video output signal conforming to a standard such as an NTSC system. Saving the graphic image from the frame memory allows an image of a next frame to be drawn in the frame memory even when the video output signal is being outputted.
Saving of the graphic image from the frame memory to the display memory is carried out after a display of a previous frame is finished. Saving from the frame memory to the display memory is carried out during VBI (Vertical Blank Interval). There is also a case where the transformation process is carried out on the graphic image that has been saved to the display memory. In this case, the conversion process must also be carried out during the VBI as well as the saving. Because saving and transformation processes that can be carried out during the VBI are limited, a size and definition of a displayable image may be determined by such the limitation. Still more, drawing to the frame memory can only be carried out after finishing the save. Therefore, drawing to the frame memory cannot be carried out asynchronously with the output.
As one measure for solving these problems, the display memory has a double-buffer structure. By using the display memory with the double-buffer structure, it becomes possible to carry out the processes such as saving of a graphic to another display memory and transformation thereof while displaying an image from one display memory on a display unit, so it becomes unnecessary to finish the saving and transformation processes during the VBI. Still more, drawing and output may be carried out asynchronously because drawing to the frame memory can be carried out immediately after finishing the saving.
However, it is not economical to provide the display memory with the double-buffer structure because storage capacity of the display memory will need at least twice the capacity as the frame buffer. Still more, there arises a problem in that latency increases.